Field of the Invention
One disclosed aspect of the embodiments relates to a solid-state image sensor, a distance measurement apparatus including the solid-state image sensor, and a camera, and, more particularly, to the solid-state image sensor for distance measurement, which is used for distance measurement of a digital still camera or a digital video camera.
Description of the Related Art
In a digital still camera or a video camera, a solid-state image sensor, in which distance measurement pixels having a distance measurement (focus detection) function are arranged in all or a part of pixels of the solid-state image sensor to measure a distance in a phase difference method, is discussed in Japanese Patent No. 4027113. The distance measurement pixel includes a plurality of photoelectric conversion elements, and is configured such that light flux passing through different exit pupil areas of a imaging lens is guided to different photoelectric conversion elements. By using the plurality of distance measurement pixels, the distance measurement function detects images by light flux passing through different exit pupil areas (an image A and an image B, respectively) to measure a deviation amount of the image A and the image B. The distance measurement function calculates a defocus amount from the deviation amount and a base length (an interval between different exit pupil areas) to measure a distance (to detect a focus position).
In this case, an exit pupil surface of the imaging lens and a surface of the photoelectric conversion element have a substantially optical conjugate relation. Therefore, an exit pupil area, through which light flux passes, is determined according to the position or size of the surface of the photoelectric conversion element. For example, when a center of gravity of the photoelectric conversion element is arranged outward from a central axis of the distance measurement pixel, the exit pupil, through which light flux passes, is shifted to the outside of the pupil. Hence, the base length is lengthened. Also, when the photoelectric conversion element is enlarged, the exit pupil area, through which light flux passes, is enlarged. Therefore, light quantity received by the photoelectric conversion element is increased and sensitivity is increased accordingly.
However, in the actual distance measurement and image capture, the optimal base length or sensitivity (the position and size of the photoelectric conversion element) varies according to luminance of an object or a imaging condition (a defocus amount). For example, when the distance is measured with high accuracy, the base length needs to be lengthened. Also, in the case of a low-luminance object, noise increases and distance accuracy worsens. Therefore, the sensitivity needs to be increased by enlarging the area of the photoelectric conversion element. However, in the conventional methods, when distance measurement pixels are designed, the base length is uniquely determined. Therefore, an optimal distance measurement could not be achieved according to the object or the photographing condition.